Jellyfish Lake, Eil Malk Island, Palau, Pacific Ocean

Jellyfish Lake is stratified into two layers, an oxygenated upper layer (mixolimnion) and a lower anoxic layer (monimolimnion). The oxygen concentration in the lake declines from about 5 ppm at the surface to zero at 15 meters (at the chemocline). Stratification is persistent and seasonal mixing does not occur. The lake is one of about 200 saline meromictic lakes that have been identified in the world. However most of these lakes are of freshwater origin. Permanently stratified marine lakes are unusual, but on Eil Malk and on other nearby islands there are eleven other apparently permanent stratified marine lakes.
The stratification of the lake is caused by conditions which prevent or restrict the mixing of water vertically. These conditions include:
1. The lake is surrounded by rock walls and trees which substantially block the wind flow across the lake that would cause mixing.
2. The primary water sources for the lake (rain, runoff and tidal flows through tunnels) are all close to the surface.